The mechanism of selective inhibition of protein synthesis resulting from viral infections has been a subject of long-standing interest. The purpose of these proposed studies is to delineate the mechanism of selective of protein synthesis using vaccinia virus as a model. Recent studies from our laboratory have indicated that poly-riboadenylic acid (poly(A)) plays a unique role in the selective inhibition of protein synthesis by vaccinia virus in an in vitro system. The present proposal will focus on the determination of 1) the role of poly(A) in in vivo selective inhibition by vaccinia virus and 2) the mechanism by which poly(A) inhibits protein synthesis. With regards to our first goal we plan to: 1. Determine whether "free poly(A)" produced in cells infected with vaccinia virus can selectively inhibit HeLa cell protein synthesis in the in vitro messenger-dependent reticulocyte system. 2. Determine whether synthetic poly(A) or poly(A) obtained from infected cells will inhibit protein synthesis in intact cells. 3. Determine whether oligo(dT) will reverse inhibition of host- cell protein synthesis when added to infected cells. With regards to the second major goal of this project we plan to: 1. Determine the kinetics of inhibition of protein synthesis by poly(A). 2. Determine the effect of poly(A) on the polyribosome pattern of reticulocyte lysates. 3. Determine the effect of poly(A) on the complex formation of initiator met-tRVA with the 40-S ribosomal subunit. 4. Determine whether radioactive poly(A) preferentially binds to host mRNAs rather than vaccinia virus mRNAs. 5. Determine the effect of purified initiation factors on the reversal on inhibitor of protein synthesis by poly(A). If poly(A) is employed by vaccinia virus to selectively inhibit host- cell protein synthesis then the understanding of the mechanisms of this cytopathological event will also have great practical value because vaccinia virus is being considered to be used as a vector to carry "foreign genes for the purpose of vaccinating animals and eventually humans.